Quadrature amplitude modulation (QAM) is a modulation technique that may be used, for example, in quadrature receivers. A received signal may be divided into two carrier waves out of phase with each other by 90°, and are thus called quadrature carriers or quadrature components. Using QAM, arbitrarily high spectral efficiencies may be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel.
In an ideal RF quadrature receiver, in-phase (I) and quadrature (Q) branches are perfectly orthogonal and have equal gain over frequency. Any imbalance from this ideal creates undesirable images since signals will be partially leaked to their frequency-negated counterparts. That is, a signal x Hz above a local oscillator (LO) will create an image perceived x Hz below LO, and simultaneously a signal x Hz below LO will create an image perceived x Hz above LO. This can have serious consequence in a wideband multi-carrier direct conversion receiver as these images fold directly into the baseband. Unless corrected, these images reduce signal-to-noise ratio (SNR), and consequently receiver sensitivity. The most serious consequences occur when magnitudes of received signals are highly dissimilar; loss of SNR is accentuated when a weak receive signal experiences interference from the image of a strong blocker.